Electroless copper plating compositions are in widespread use in metallization industries for depositing copper on various types of substrates. In the manufacture of printed wiring boards, for example, the electroless copper baths are used to deposit copper into through-holes and circuit paths as a base for subsequent electrolytic copper plating. Electroless copper plating also is used in the decorative plastics industry for deposition of copper onto non-conductive surfaces as a base for further plating of copper, nickel, gold, silver and other metals as required. Typical electroless plating baths which are in commercial use today contain divalent copper compounds, chelating agents or complexing agents for the divalent copper ions, formaldehyde reducing agents and various addition agents to make the bath more stable, adjust the plating rate and brighten the copper deposit. Although many of such baths are successful and are widely used, the metallization industry has been searching for alternative electroless copper plating baths that do not contain formaldehyde due to its toxic nature.
Formaldehyde is known as an eye, nose and upper respiratory tract irritant. Animal studies have shown that formaldehyde is an in vitro mutagen. According to a WATCH committee report (WATCH/2005/06—Working group on Action to Control Chemicals—sub committee with UK Health and Safety Commission) over fifty epidemiological studies have been conducted prior to 2000 suggested a link between formaldehyde and nasopharyngeal/nasal cancer but were not conclusive. However, more recent studies conducted by IARC (International Agency for Research on Cancer) in the U.S.A. showed that there was sufficient epidemiological evidence that formaldehyde causes nasopharyngeal cancer in humans. As a result the INRS, a French agency, has submitted a proposal to the European Community Classification and Labelling Work Group to reclassify formaldehyde from a category 3 to a category 1 carcinogen. This would make usage and handling of it more restricted, including in electroless copper formulations. Accordingly, there is a need in the metallization industry for a comparable or improved reducing agent to replace formaldehyde. Such a reducing agent must be compatible with existing electroless copper processes; provide desired capability and reliability and meet cost targets.
Hypophosphites have been suggested as a replacement for formaldehyde; however, plating rates of baths containing this compound are generally too slow.
U.S. Pat. No. 5,897,692 discloses formaldehyde free electroless plating solutions. Compounds such as boron hydride salts and dimethylamine borane (DMAB) are included as reducing agents. However, such boron containing compounds have been tried with varying degrees of success. Further, these compounds are more expensive than formaldehyde and also have health and safety issues. DMAB is toxic. Additionally, resultant borates have adverse effects on crops on release into the environment.
Accordingly, there is still a need for an electroless copper bath which includes alternative reducing agents to replace many of the conventional reducing agents currently used in the electroless plating industry that is stable, provides acceptable copper deposits and is environmentally friendly.